[407] SCIENTIFIC MANUAL. 121 



CHAPTER X. 



FARM DRAINAGE. 



This consists in the removal of surplus water from agri- 

 cultural lands by means either of open ditches, or cov- 

 ered conduits, constructed a few feet below the surface of 

 the ground. 



The natural drainage of some soils is sufficient to carry 

 off promptly all surplus water without the aid of artificial 

 means, but where this natural drainage is, to a large extent, 

 from the surface, it is extremely injurious, and often ruin- 

 ous to the land. 



If, however, the subsoil is sufficiently pervious to allow 

 the surplus water to sink rapidly below the ordinary range 

 of the roots of plants, and thence to pass freely off to 

 streams, the natural drainage is perfect, and favorable to 

 the growth of vegetation, unless both soil and subsoil 

 are too coarse to admit of the rise of moisture by capillary 

 attraction, and a reasonable atomic absorption and reten- 

 tion. 



If the soil and subsoil are clay or loam, and the natural 

 drainage perfect, we have the necessary conditions prece- 

 dent to "high farming." 



In cases where artificial drainage is necessary, the first 

 thing to be considered is 



HOW TO START. 







This will depend upon the source of the evil, the topog- 

 raphy of the surface, and the character of the subterranean 

 strata. 



The source of the evil on bottom lands is often found in 

 numerous small springs at the base of the adjacent hills, 

 none large enough to justify open ditches or to make use- 

 ful springs. In such cases an impervious stratum of clay 

 is usually found a few feet below the surface, which pre- 

 vents the water from passing off below, and consequently 



